To make the chocolate cupcakes:

• Preheat the oven to 350°F. Line 10 standard muffin tins with paper liners and lightly spritz each liner with non-stick spray (NOTE: using the non-stick spray is optional, but I find it helps the liners come off the cupcakes more cleanly!)
• In a large bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, kosher salt, and espresso powder; set aside for now.
• In a separate medium-sized bowl, whisk together the buttermilk, sour cream, vegetable oil, egg, and vanilla extract.
• Pour the wet ingredients into the bowl with the dry ingredients, and use a rubber spatula to gently fold the ingredients together until combined. The batter will be thick.
• Next, carefully add the hot coffee to the batter and continue to stir using the rubber spatula until a thin, smooth batter forms.
• Divide the batter evenly between the prepared muffin tins, filling each tin about 2/3 of the way full (NOTE: I like to use a spring loaded cookie scoop for this!). Bake until a toothpick inserted into the center of one of the cupcakes comes out clean or with a few moist crumbs attached, 14 to 16 minutes. Allow the cupcakes to cool in their tins for 5 minutes before carefully removing to a wire rack to cool completely.

To make the chocolate buttercream:

• In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth and creamy, about 1 minute. Scrape down the sides of the bowl. Add the powdered sugar, cocoa powder, vanilla, kosher salt, and heavy cream to the bowl. (NOTE: before the next step, I like to place a clean kitchen towel over the mixer to prevent ingredients from flying everywhere).
• Turn the mixer on low speed and gradually increase to medium, mixing until the ingredients come together. Once the ingredients have come together, increase the mixer to high-speed and beat for 1 minute, until the buttercream is light and fluffy. Scrape down the sides and bottom of the bowl. At this point, check the consistency of your buttercream: add more heavy cream, 1 tablespoon at a time, if necessary to thin out the buttercream to a piping consistency.
• Reduce the mixer to low speed and beat the buttercream for 1 minute, until smooth and creamy. Use immediately.

Assembly:

• Transfer the buttercream to a piping bag fitted with a large tip (NOTE: I used an Ateco 828 large open star tip). Pipe the buttercream onto the cooled cupcakes. Alternatively, spread the buttercream on top of each cooled cupcake using an offset spatula. Top with sprinkles, if desired.